1. Field of the Invention
The present invention relates to a semiconductor device provided with a conductivity modulation type MISFET, in which all the electrodes are formed on one surface thereof.
2. Description of the Related Art
A conventional semiconductor device provided with a conductivity modulation type MISFET (IGBT) comprises, as shown in FIG. 8, an n.sup.+ -type buffer layer 2 and an n.sup.+ -type conductivity modulation layer 3 as a drain layer respectively sequentially stacked on a p.sup.+ -type minority carrier injection layer 1, a p.sup.+ -type base region 6 and an n.sup.+ -type source region 7 respectively diffused and formed by self-alignment in the surface side of the conductivity modulating layer 3 by use of a polysilicon gate 5 oxide film 4 as a mask, a gate electrode 8 connected onto the polysilicon gate 5, a p.sup.+ -type contact region 9 of the p.sup.+ -type base region 6, a source electrode 10 connected with the n.sup.+ -type source region 7, and a drain electrode 11 provided as a coat over the minority carrier injection layer 1 in the back side.
Through an inversion layer formed in the surface of the p.sup.+ -type base 6, electrons (majority carriers) flow through the conductivity modulating layer 3 in the vertical direction as illustrated by the solid line arrow, and at the same time positive holes (minority carriers) are injected into the conductivity modulating layer 3 as illustrated by the dotted line arrow. Consequently, a conductivity modulating condition is induced, in turn lowering resistance in the conductivity modulating layer 3. It is therefore possible to obtain a large current capacity.
Although there is no problem in a single such element, there are the following problems in a semiconductor device in which a plurality of IGBT elements or other elements are formed on one and the same semiconductor substrate.
That is, for a conventional IGBT element having a gate electrode 8 and a source electrode 10 in its surface side and a drain electrode 11 provided as a coating on its back side, it is necessary to provide a technique of isolation between elements such as isolation of pn junctions and so on, both on the back side as well as on the principal surface, so that there is a serious problem in technique as well as in cost. Moreover, even if this problem could be solved, wiring between respective elements would extend between the opposite surfaces, so that it would be difficult to carry out the wiring.